1. Field of the Invention
The present invention applies to the field of wireless communications systems and, in particular, to modifying hopping sequences used for communications channels.
2. Description of the Prior Art
Mobile radio communications systems such as cellular voice and data radio systems typically have several base stations in different locations available for use by mobile or fixed user terminals, such as cellular telephones or wireless web devices. Each base station typically is assigned a subset of the frequencies available to the network operator. This subset is the “physical carrier set” available to the base station to use for communications with user terminals. This physical carrier set may have physical carriers that are different from those of neighboring base stations in order to avoid interference between neighboring base stations.
Each base station can organize the set of physical carriers into logical channels, i.e., traffic and overhead channels. For example, a logical channel may consist of a time slot in a TDMA (Time Division Multiple Access) frame on a physical carrier. A TDMA frame may contain, for example, eight downlink transmit time slots followed by eight uplink receive time slots. A physical carrier may be a 200 kHz band around a central frequency, such as 800 MHz or 1.9 GHz. This band represents a physical carrier frequency used by the base station and its user terminals for communication. Thus, a base station transmits to a given user terminal, for example, on the second transmit and receive time slots on this carrier frequency in a given frame. Furthermore, the logical channel may be organized using common techniques, such as FDD (Frequency Division Duplex), TDD (Time Division Duplex), FDMA (Frequency Division Multiple Access), and CDMA (Code Division Multiple Access).
Logical channels may further be defined by a hopping sequence. A hopping sequence indicates the sequence of resources used by a logical channel. For example, the hopping sequence may indicate that the logical channel hops carrier frequencies periodically. Frequency hopping is often used to counter fading and radio frequency (RF) interference. Fading due to multi-path propagation in the RF environment is generally frequency dependent, with different carriers experiencing different levels of fading. Frequency hopping can average the fading for many user terminals, so that no one terminal suffers severe fading. Frequency hopping can also serve to average interference among user terminals, since with each hop, each user terminal faces a different set of interferers of various intensities. Thus, in the long run, each user terminal experiences similar levels of interference, and no user terminal experiences severe interference.
Typically, the hopping sequences used by a wireless network to define logical channels are related in some way. A plurality of relating hopping sequences is referred to here as a family of hopping sequences. Families of hopping sequences are typically designed to have one or more desirable qualities. There are numerous ways of generating such families of hopping sequences. However, these families generally have predetermined sizes. For example, families generated using Orthogonal Latin Squares must have a prime power number of sequences.